Not everything can have a “slider” option. When the very foundation of single player versus how a server is rendered in the game world is completely different, a slider isn’t exactly going to be an option.
The developers used several tricks in order for lower end (and consoles) to support the game as a single player option, this is just yet another example of those tricks in action. If your computer can handle it (as a multiplayer server but just for you), you can set it up as a server. Most higher end machines with a good amount of RAM and a nice CPU to back it up can handle both the game and the server just fine.